As for conventional screw press type filters, there is, for example, a through-screw type filter cylinder system, as disclosed in Japanese Patent Publication No. SHO 63-65364 (U.S. Pat. No. 1,520,106). A filter of this system comprises a drum, a filter cylinder installed in a substantial length excluding the opposite ends of the drum for filtering raw water to separately migrate the water content thereof radially from inside to outside of the drum, a number of through-holes formed in the peripheral surface of the drum in said substantial length, a screw installed throughout the length of the drum to extend through said filter cylinder, the spaces within said drum at the opposite ends thereof being used as an inlet chamber for raw water and an outlet chamber for dehydrated cake. The filtration passage in the filter cylinder is defined by small clearances between alternating annular fixed and movable plates that constitute said cylinder.
In the above arrangement, normally as the water content is pressed out from the clearances between the plates toward the outer periphery of the filter cylinder, the volume of an object material to be processed (the residue remaining after the pressing of the raw water) that is pressed against the screw vanes corresponding to one pitch is progressively reduced according to the amount of water pressed out. In contrast thereto, in the usual construction wherein the volume ratio (processing volume/unit length) and the screw pitch are constant throughout the length of the filter cylinder, the screw load becomes lighter as the rear end of the filter cylinder is approached, resulting in the rear end of the screw feeding no substantial load.